1. Field of the Invention
The invention relates generally to a glass composition, more particularly, to a Tm3+-doped fluorophosphate glass material that provides amplification and/or laser action for at least one wavelength of light in the range of about 1450 nm to about 1530 nm.
2. Technical Background
In optical telecommunications networks, high bandwidth is desired for applications such as the Internet, video on demand, and videophone. In many optical communications systems, optical signals having wavelengths in the range 1530-1560 nanometers (nm) are utilized. This wavelength range corresponds to the xe2x80x9cC-bandxe2x80x9d in telecommunications. This wavelength range also corresponds to a minimum attenuation region for silica and silica-based fibers.
Optical amplifiers are utilized to amplify the optical signals in those wavelength regions. Conventional optical amplifiers for telecommunications include erbium (Er)-doped silicate glass. The Er-doped silicate glass optical amplifier operates in the C-band and can also amplify optical signals in the 1570 nm -1620 nm range (also referred to as the L-band).
In order to increase optical bandwidth for telecommunications, more wavelengths will need to be transmitted. One wavelength range of interest is the 1450 nm-1530 nm wavelength band, often referred to as the xe2x80x9cS-band.xe2x80x9d However, this wavelength band is outside of the Er-based material amplification range.
Within the 1450 nm-1530 nm wavelength band, trivalent thulium (Tm3+) has an emission band centered at about 1480 nm, corresponding to the 3F4-3H4 transition in Tm3+. As there is some confusion in the literature as to the identity of the upper level of this transition (either 3F4 or 3H4), this transition will be referenced to herein as the xe2x80x9c1480 nm transitionxe2x80x9d and the upper level will be referenced as the 3F4 level. In order to generate a population in the upper 3F4 energy level, for example, radiation at 780 nm to 800 nm is absorbed by the Tm3+ material, whereby ions are transferred to the 3F4 excited state from the 3H6 ground state. Other types of pumping schemes, which rely on different energy transfer and up-conversion mechanisms, can also be utilized.
Attempts have been made to provide optical fibers that provide gain, amplification, or laser action for at least one wavelength of light in the range of about 1400 nm to about 1530 nm. The glass ZBLAN (an acronym for ZrF4xe2x80x94BaF2xe2x80x94LaF3xe2x80x94AlF3xe2x80x94NaF) is one such example that provides for an increased lifetime (of 1 or more milliseconds) for the upper level of the 1480 nm transition. Fluoride glasses such as ZBLAN, however, have several undesirable properties as will be described below.
One problem with fluoride glasses is relatively low durability as compared to non-fluoride glasses (e.g. silica-based glass, antimony silicate glass). Hermetic packaging is thus required to increase the durability of ZBLAN because polyacrylate-coated ZBLAN fiber generally fails the stringent Telecordia specifications for durability under damp heat and temperature cycling without the hermetic packaging. Unfortunately, due to the high cost of hermetic packaging, efforts to commercialize fluoride glasses such as ZBLAN-based fiber have not been successful.
Another problem with fluoride glasses in general is that they are prone to devitrification (i.e., the break down of surface elements of the glass) during reheating, which is required in optical fiber formation. Thus, it follows that fluoride glasses are difficult to fabricate into long lengths of high-quality, high-strength fiber. ZBLAN-based fibers, for example, can have a strength as high as about 500 MPa to about 580 MPa, (70-80 kpsi) as compared to about 2000 MPa to about 3000 MPa for antimony silicate or Al-doped silica fibers. Furthermore, it is difficult to draw long lengths of fluoride glasses because small preform diameters are required, as fluoride glasses are generally too unstable to cast into large objects.
Another problem with ZBLAN is that zirconium is prone to reduction (which can result in the formation of metallic deposits on the surface of the glass) when melted under inert atmosphere (e.g., under argon or nitrogen). As a result, ZBLAN and related fluorozirconate and fluoroindate glasses must be melted under so-called reactive atmospheric conditions (e.g., in the presence of chlorine, SF4, SF6, etc.). Reactive atmospheric conditioned manufacturing processes, however, are generally very complex and expensive, as reactive atmospheres constrain the types of crucibles, heating elements, etc. that can be used to manufacture the optical fiber, and tend to limit the maximum size of the batch that can be handled.
Optical fibers based on antimony silicate (MCS) glasses have been developed as an alternative to ZBLAN. In MCS materials, TM3+ ions are somewhat isolated from direct interactions with the silicate host glass by forming coordination environments consisting almost entirely of SbOx anions. Also, antimony is considerably heavier than silicon and forms slightly weaker bonds with oxygen than silicon. As a result, Tm3+ emission lifetimes are lower, resulting in poor power conversion efficiency relative to ZBLAN.
At least one contributing factor to MCS fiber""s relatively poor performance in the range of about 1450 nm to about 1530 nm is related to a phenomenon known as photodarkening. Photodarkening is produced by cascaded up-conversion (i.e., multiple absorption and promotion of electrons up to a plurality of electronic states) between equivalently spaced energy levels to higher energy levels. The cascaded upconversion in MCS fiber is strong in the range of about 1400 nm to about 1530 nm, whereby light in that wavelength band propagating along the optical fiber is absorbed. At the high pump power densities of, for example, single mode fibers operating in the 1400 nm to 1530 nm transmission wavelength band, cascaded up-conversion can lead to visible color center formation in the glass. These color centers manifest themselves as a large decrease in optical fiber transparency at very short wavelengths with a broad tail extending well into the infrared (IR) (i.e., at the desired propagation wavelength(s)).
Thus, a need exists for a low cost optical fiber having suitable 1450 nm to 1530 nm emission lifetimes, yet also with sufficient thermal stability and durability, as well as minimized photodarkening characteristics.
In one aspect of the present invention, a glass material comprises a fluorophosphate glass having a non-zero concentration of Tm3+. The fluorophosphate glass further includes cation elements that include at least an alkaline earth, phosphorus, and aluminum, and anion elements that include oxygen (O) and fluorine (F). In the fluorophosphate glass, a ratio of F/(F+O) is in the range of about 0.50 to about 0.8.
In another aspect of the present invention, a fluorophosphate glass composition comprises about 13 cation % to about 26 cation % P, less than about 10 mol % alkali metal halide, and a concentration of Tm3+ ions at an amount where the lifetime of an upper level for a 1480 nm transition is at least about 350 xcexcs.
In another aspect of the present invention, an alkali-free fluorophosphate glass is provided having a concentration of Tm3+ ions at an amount where the lifetime of an upper level for a 1480 nm transition is at least about 350 xcexcs.
In another aspect of the present invention, an alkali-free fluorophosphate glass comprises about 13 cation % to about 26 cation % P, and about 40 cation % to about 75 cation % R, where R comprises at least one alkaline earth selected from at least one of Mg, Ca, Sr, and Ba. The fluorophosphate glass further comprises about 13 cation % to about 40 cation % Al, and 0.001 cation % to about 0.5 cation % Tm.
In another aspect of the present invention, an optical amplifier is provided that comprises a Tm-doped fluorophosphate glass having the constituents described above.
The present invention provides Tm-doped compositions that enable the construction of optical fibers having suitable 1450-1530 nm emission lifetimes, acceptable thermal stability and durability, and mimimal photodarkening characteristics. Other advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.